1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly, to an improvement of an antireflection coating formed on an interconnection layer for preventing reflection of exposure light used at the time of photolithography of an interconnection.
2. Description of the Prior Art
FIG. 1 is a cross sectional view showing schematically a structure of a contact hole portion of a conventional semiconductor device. In FIG. 1, the conventional semiconductor device comprises a semiconductor substrate 1 of silicon, an impurity diffused layer 2 formed in a predetermined region on the surface of the semiconductor substrate 1 to serve as an active region, an underlayer insulating film 3 comprising a silicon oxide film or the like formed for the purpose of protecting and stabilizing the surface of the semiconductor substrate 1, an interconnection of an aluminum-silicon alloy (referred to as Al-Si alloy interconnection hereinafter) 4 formed in a predetermined region on the underlayer insulating film 3 and electrically connected to the impurity diffused layer 2 through a contact hole 10, and an antireflection coating 5 of an aluminum-silicon alloy formed on the Al-Si alloy interconnection 4 for preventing reflection of exposure light irradiated at the time of photolithography for patterning the Al-Si alloy interconnection 4 in a predetermined shape. Referring now to FIG. 1, a method for forming the interconnection of the conventional semiconductor device will be briefly described.
After a resist film having a predetermined shape is formed on the surface of the semiconductor substrate 1 using a photolithographic technique, an impurity diffused layer 2 is formed in a predetermined region of the surface of the semiconductor substrate 1 using an ion implantation process or the like utilizing this patterned resist film as a mask. An underlayer insulating film 3 comprising a silicon oxide film or the like is then formed on the exposed entire surface using a CVD process or the like for the purpose of protecting and stabilizing the surface of the semiconductor substrate 1. A contact hole 10 is then formed in a predetermined region of the underlayer insulating film 3 using photolithographic and etching techniques in order to make electrical connection to the impurity diffused layer 2. A thin film of an Al-Si alloy containing 0.5 to 2.0% silicon by weight is then formed on the underlayer insulating film 3 and the exposed surface of the semiconductor substrate 1 using a spattering process or the like.
Additionally, in order to prevent reflection of exposure light irradiated in a photolithographic process for patterning a thin film of an alloy on the thin film of an Al-Si alloy in a predetermined shape to form an interconnection, a thin film 5 of Al-Si alloy, 100 to 500.ANG. in thickness, containing 20 to 50% silicon by weight, is formed using an evaporation process for the purpose of reducing reflectivity to the exposure light. Thereafter, etching is made utilizing as a mask a resist film (not shown) patterned in a predetermined shape by a photolithographic process, to form the Al-Si alloy interconnection 4 having a predetermined shape. It is to prevent occurrence of an alloy pit caused as a result of the reaction between aluminum of the interconnection and the impurity diffused layer in a heat-treating process for achieving good ohmic contact of the interconnection with the impurity diffused layer that the Al-Si alloy is used as an interconnection material. This alloy pit causes "punch through" of the diffused layer when the depth of the diffused layer is small, which causes an electrical short between the substrate and the interconnection.
The antireflection coating having a thickness of approximately 100 to 500.ANG. is formed to prevent exposure light from being reflected from the thin film of an alloy of an underlayer and thus even a photoresist film in an undesired portion from being exposed in the photolithographic process of a photoresist film, thereby to obtain a desired patterning precision.
The Al-Si alloy interconnection 4 formed in the above described manner is generally heat-treated for about several ten minutes at a temperature of 400 to 500.degree. C. in an atmosphere of nitrogen or hydrogen in order to achieve good ohmic contact between the alloy interconnection 4 and the impurity diffused layer 2. In this heat-treating process, silicon in the antireflection coating 5 containing silicon at a high concentration diffuses into the Al-Si alloy interconnection 4, so that the concentration of silicon in the alloy interconnection 4 is increased. As a result, the concentration of silicon in the alloy interconnection 4 exceeds the limit of solid solubility, so that silicon is easily deposited in the interface of the alloy interconnection 4 and the semiconductor substrate 1 of silicon by solid phase epitaxial growth in which substrate silicon is used as a seed crystal. This deposited silicon 6 is close to an intrinsic semiconductor and has a high specific resistance value. Thus, if the deposited silicon 6 is formed in a part or all of the contact hole 10, contact resistance between the alloy interconnection 4 and the impurity diffused layer 2 becomes high, which causes electrical failures.
Furthermore, in this heat-treating process, silicon which exceeds the limit of solid solubility is deposited in the Al-Si alloy interconnection 4 on the underlayer insulating film 3 comprising a silicon oxide film, so that a mass of silicon referred to as a silicon nodule 7 is formed. This silicon nodule 7 is also close to the intrinsic semiconductor and has a very high specific resistance value. In addition, the silicon nodule grows to the size of approximately 1.mu.m. Thus, the effective cross-sectional area of the interconnection becomes relatively small and the current density in this portion is substantially increased, so that failures such as disconnection due to electromigration are liable to occur, thereby to decrease electromigration resistance of the interconnection.
More specifically, in the conventional semiconductor device, the film of an aluminum-silicon alloy containing 20 to 50% silicon by weight is used as an antireflection coating for preventing reflection of exposure light at the time of photolithography for forming the Al-Si alloy interconnection. Thus, by heat treatment for achieving good ohmic contact between the interconnection and the impurity diffused layer, silicon may be deposited in the contact hole portion by solid phase epitaxial growth and the silicon nodule may occur in the alloy interconnection on the underlayer insulating film by deposition of silicon, so that electrical failures caused by the increase in contact resistance occur and disconnection of the interconnection due to electromigration occurs.